Methods and devices for magnetically transferring indicia to a wet coating composition are already known as such in the art, for instance from International applications Nos. WO 2004/007095, WO 2004/007096, WO 2005/000585, WO 2005/002866 and European patent application No. EP 1 650 042.
According to these methods, a layer of coating composition, such as an ink or varnish, is first applied to at least a part of the surface of a substrate, which coating composition comprises at least one type of magnetic or magnetizable particles. While the layer of coating composition is still wet, the layer is exposed to a determined magnetic field generated at a surface of a magnetic-field-generating device, thereby orienting the magnetic or magnetizable particles along field lines of the magnetic field. The layer of coating composition is then dried or cured, thereby fixing the orientation of the magnetic or magnetizable particles.
European patent application No. EP 1 787 728 (which application was published only after the priority date of the present application) discloses a magnetic plate for printing of optical effects, which plate comprises a magnetizable composite material that is selectively magnetized so that one or more first regions across the surface of the plate provide a first magnetic field having a predetermined direction. These first regions form a logo, indicia or image of an object. Magnetic material in one or more other second regions surrounding the first region are either unmagnetized or magnetized differently from the one or more first regions so as to provide a contrast in magnetic field.
Magnetic or magnetizable particles (also designated as “magnetic flakes”), which have the particularity that they can be oriented or aligned by an appropriately-applied magnetic field, are discussed in particular in U.S. Pat. No. 4,838,648, European patent application EP 0 686 675, and International applications WO 02/073250, WO 03/000801, WO 2004/007095, WO 2004/007096 and WO 2005/002866.
Such particles or flakes are in particular used as optically-variable pigments in so-called optically-variable inks, or OVI®'s (OVI® is a registered trademark of SICPA Holding SA, Switzerland) to produce high-level security patterns, especially for banknotes.
The most convenient method to apply the above magnetic flakes is by silk-screen printing as discussed in the above-mentioned International application WO 2005/000585. This is mainly due to the fact that the flakes have a relatively important size which restricts the choice of available printing processes for applying inks or varnishes containing such flakes. In particular, one has to ensure that the flakes are not destroyed or damaged during the printing process, and silk-screen printing constitutes the most convenient printing process to achieve this goal. Furthermore, silk-screen printing has the advantage that the inks or varnishes used in such a process exhibit a relatively low viscosity which favours proper orientation of the magnetic flakes.
Nevertheless, other printing processes could be envisaged to apply coating compositions containing magnetic flakes, such as flexographic printing or gravure printing. In European patent application EP 1 650 042, it is even proposed to apply such magnetic flakes in an intaglio printing process, whereby the paste-like intaglio ink containing the magnetic pigments is heated to decrease the viscosity of the ink and thereby allow the flakes to be oriented more easily with a magnetic or electric field. This can be performed in a conventional intaglio printing press, since the plate cylinder of such presses is commonly brought to an operating temperature of approximately 60 to 80° C. during printing operations.
Orientation of the magnetic flakes contained in the wet coating composition is carried out by applying an adequate magnetic field to the freshly-applied layer of coating composition. By appropriately shaping the field lines of the magnetic field, the magnetic flakes can be aligned in any desired pattern producing a corresponding optically-variable effect which is very difficult, if not impossible to counterfeit. An adequate solution for orienting the magnetic flakes, as discussed in International application WO 2005/000585 consists in bringing sheets carrying layers of wet coating composition in contact with a rotating cylinder carrying a plurality of magnetic-field-generating devices.
International application WO 2005/002866, which is incorporated herein by reference, discloses a particular type of magnetic-field-generating device comprising a body, such as a flat plate or a cylindrically curved plate, made of a permanent magnetic material which is permanently magnetized in a direction substantially perpendicular to a surface of the body. The said surface of the body furthermore carries indicia in the form of engravings causing perturbations of its magnetic field.
FIG. 1 is a schematic cross-sectional view of a magnetic field simulation taken from the above-mentioned International application which illustrates an example of a vertically magnetized permanent magnetic plate, designated by numerical reference 1, comprising a rectangular engraving 2. In this example, the engraved plate 1 is made of Plastoferrite (such as the Plastoferrite model M100.8 sold by Maurer Magnetic AG, CH-8627 Grüningen, http://www.maurermagnetic.ch) magnetized in a direction perpendicular to the surface of the plate 1.
As illustrated in FIG. 1, as the permanent magnetic body 1 is vertically magnetized, the field lines of the magnetic field are mostly vertical in the region of the surface of the body, except in the region of the vertical walls of the engraving 2. This implies that most of the magnetic pigments contained in the wet composition are aligned in a vertical manner, perpendicularly to the surface of the substrate. In other words, considering the fact that the pigments are mostly reflective when they are aligned substantially horizontally, the resulting pattern induced in the coating composition by means of the device of FIG. 1 is mostly not reflective, when seen and illuminated perpendicularly to the surface of the substrate.
FIGS. 2a to 2c are greyscale photographs, taken along three different viewing angles, of a magnetically-induced pattern representing the value “50” within an oval shape which was produced by means of a device according to the principle of International application WO 2005/002866 illustrated in FIG. 1. More precisely, the pattern was produced using a body having engravings representing the value “50” within an engraved oval shape.
The layer of coating composition was applied with a silk-screen printing process on top of a black offset background using an OVI® silk-screen ink comprising gold-to-green optically variable magnetic pigment corresponding to the 7-layer pigment design disclosed in WO 02/73250. The purpose of the black (or dark) offset background is to increase the contrast in the induced pattern by making the reflective parts of the pattern (i.e. the portions where the pigments are oriented substantially horizontally with respect to the surface of the substrate) stand out as compared to the less reflective parts of the pattern (i.e. the portions where the pigments are oriented substantially vertically with respect to the surface of the substrate, thereby revealing the underlying background).
As already mentioned hereinabove, most part of the induced pattern produced according to the known method disclosed in International application WO 2005/002866 is relatively dark, i.e. the pigments are mostly aligned vertically, thereby making the dark offset background visible through the layer of coating composition. As can be seen from the photographs of FIGS. 2a to 2c, the most reflective portions of the magnetically-induced pattern correspond to the position of the walls of the engravings. Looking at the pattern, one basically has the impression that the oval shape and the value “50” stand out in relief above the background as illustrated in FIGS. 2a to 2c. 
The patterns that can be produced according to the known method disclosed in International application WO 2005/002866 discussed above are already quite an improvement as compared to the patterns that could previously be produced. A need has nevertheless arisen for an improved approach which would enable to produce different patterns, albeit with comparable means, especially patterns exhibiting a comparatively lighter, more reflective optical effect.
A distinct approach is proposed in International application No. WO 2006/114289 which discloses a method for creating color effect images on a carrier substrate. According to this method, a latent magnetic image comprising magnetic pixels and non-magnetic pixels is created on a magnetizable printing form. A carrier substrate provided with a decorative layer containing non-spherical, preferably needle shaped or lamellar magnetic color effect pigments is guided past the magnetizable printing form such that the orientation of color effect pigments of the decorative layer relative to the carrier substrate changes with the aid of the images of the field lines created by the magnetic pixels of the magnetizable printing form. The color pigments are ultimately fixed in the decorative layer with the orientation thereof modified by the magnetizable printing form.
According to WO 2006/114289, the magnetizable printing form comprises a soft magnetic band and electromagnetic printing heads are used to locally change the magnetic coercivity of the soft magnetic band to form the desired magnetic pixels. A “soft magnetic” material is commonly understood as designating a magnetizable material which has the ability to lose its memory of previous magnetizations, as opposed to “hard” or “permanent” magnetic material which stay magnetized for a long time. According to WO 2006/114289, each magnetic pixels thus acts as an elementary magnet locally affecting the orientation of the field lines of the magnetic field.
This approach is fundamentally different from that of WO 2005/002866 in that it is not based on the use of an engraved body for influencing the orientation of the field lines of a magnetic field generated by separate electromagnetic means. Moreover, since a soft magnetic material is used according to WO 2006/114289, there is a high risk that the magnetic configuration of the magnetizable body may become lost or be affected by external magnetic fields. The solutions of WO 2005/002866 and WO 2006/114289 may not therefore be combined together. The solution of WO 2006/114289 is furthermore less robust than that of WO 2005/002866 and is thus not suited for use in a conventional production environment such as that of a printing plant and/or for implementation thereof on a printing press.